Antiphospholipid antibodies are a heterogenous family of antibodies that recognize and bind to a variety of phospholipid/protein complexes. These antibodies are associated with an increased frequency of thromboembolic complications (e.g., deep venous thrombosis, pulmonary embolism, stroke, or myocardial infarction), recurrent fetal deaths, and thrombocytopenia. Although several investigators have demonstrated that family members of patients with antiphospholipid antibodies have an increased frequency of these and other autoimmune disorders, very little information is available on the genetics of this disorder. In addition, fewer than half of the patients with antiphospholipid antibodies will ever manifest a clinical event, but there are no unique aspects of these subsets of patients that have been identified to date. We have undertaken a systematic approach to understand the molecular mechanisms underlying the diverse pattern of clinical phenotypes and laboratory findings that have been observed in patients with antiphospholipid antibodies. To accomplish this, we have established a database of all individuals identified with a positive lupus anticoagulant and/or anticardiolipin antibody at Duke during the last four years. To date, we have identified over 430 patients with antiphospholipid antibodies on whom prospective clinical laboratory and outcomes data are being collected. In addition, we have also enrolled family members from 20 of these patients, obtaining genomic DNA, plasma, and serum for characterization.